1. Field of the Invention
The present invention relates to a document reader for converting image information to an electric signal by scanning a document by an image pick-up device such as a charge storage type of photo-electric converter.
2. Description of the Prior Art
A prior art document reader of the type described above is shown in FIG. 1, in which numeral 1 denotes a fluorescent lamp drive circuit which is fed from an A.C. power supply, numeral 2 denotes a document illumination fluorescent lamp energized by the drive circuit 1, numeral 3 denotes a document illuminated by a light from the fluorescent lamp 2, numeral 4 denotes a lens system for focusing a reflected light including pixel information on the document 3 illuminated by the fluorescent lamp 2 onto a photosensing plane of an image pick-up device 5, numeral 6 denotes an image signal amplifier for amplifying an image signal produced by the photo-electric conversion by the image pick-up device 5, numeral 7 denotes an A/D converter for converting the image signal to a digital image signal, and numeral 8 denotes an automatic gain control circuit for compensating a variation of an output level of the digital image signal from the A/D converter 7. The level of the photo-electrically converted digital signal is kept constant by the automatic gain control circuit 8 and an image signal is sequentially taken out therefrom.
When the pixel information of the document 3 is photo-electrically converted, a signal output level changes depending on a luminance of the fluorescent lamp 2 and a background density of the document, and significantly varies depending on an amplification of the amplifier 6. The automatic gain control circuit 8 is used to compensate for the variation of the signal output level. For example, the background density of the document bearing the pixel information, that is, a white background area of the document is detected and the output signal level is kept constant based on the signal level of the white background area. An ordinary document includes the white area in each of scan lines and the signal level is maximum in the white area. Thus, the variation of the image signal output level is compensated by keeping the maximum value of the photo-electrically converted signal at a constant level.
However, when a document including a half-tone image in an entire area such as a photograph is to be read, a reference background area (white area) which an ordinary document includes is not present and a maximum level of the photo-electrically converted signal may change from scan to scan. Accordingly, if the level of the photo-electrically converted signal is controlled by the prior art method, the density of the reproduced image varies locally and an exact half-tone image is not reproduced. Accordingly, for the half-tone image such as the photograph, it is necessary to keep the amplification factor of the photo-electrically converted signal at a constant level during the reading of the document. If a surrounding temperature or a power supply voltage changes or the luminance of the fluorescent lamp changes by degradation during this period, the level of the photo-electrically converted signal varies and the image is not exactly reproduced.
Further, the reading operation of the image pick-up device is influenced by the light adjustment operation of the light source and a nonuniform image may be reproduced.